This invention pertains to apparatus for automating the administration and management of intravenous regional anesthesia (IVRA) for both upper and lower limbs. IVRA is an alternative to general anesthesia for limb surgery. IVRA has proven to be a simple and useful technique for satisfactorily anesthetizing the upper limb and is potentially well suited for greatly expanded utilization in surgery of lower limbs and in outpatient settings. In these settings, which are rapidly increasing in number worldwide, there is a large and unmet need for a rapid, simple, safe, and reliable technique for establishing limb anesthesia. However, significant practical problems with the technology of IVRA in the prior art, considerable variations in skill involving the manual administration of IVRA, and lingering concerns over the potential toxicity of certain IVRA agents, particularly for lower limbs, have greatly limited the acceptance of this promising technique.
IVRA is an anesthetic technique which requires the use of surgical pneumatic tourniquet. Surgical pneumatic tourniquet systems are frequently used on the upper and lower limbs to help maintain a bloodless operative field by regulating the maximum pressure applied to the limb by an encircling cuff at a pressure sufficient to stop arterial blood flow past the cuff for the duration of a surgical procedure. During operations performed under IVRA, the pneumatic tourniquet serves an additional role of preventing local anesthetic agent introduced into the veins in the limb distal to the cuff from flowing proximally past the cuff and out of the limb into the circulatory system. An insufficient pressure in the tourniquet cuff soon after introduction of the local anesthetic agent into the limb may result in the anesthetic agent entering the circulatory system in a high concentration, which can cause serious adverse reactions such as cardiovascular collapse, respiratory depression, epileptic seizures or even death.
IVRA is typically administered as follows. Blood is first exsanguinated from the limb, often by wrapping the limb with an elastic bandage, beginning distally and wrapping tightly towards the heart; after exsanguination, a tourniquet cuff is applied proximal to the operative site and inflated to a predetermined cuff pressure. The elastic bandage is removed and an anesthetic agent such as lidocaine mixed with sterile saline is introduced into a vein in the limb through an intravenous cannula. The anesthetic fluid mixture remains in the veins in the limb as long as the tourniquet is inflated to a sufficient pressure. Premature release of the agent shortly after introduction, as well as leakage of the agent under the cuff during introduction or during surgery, are serious and recognized hazards associated with prior art devices used for IVRA.
Administration of IVRA may involve the use of a single-bladder or a dual-bladder tourniquet cuff. If a dual-bladder cuff has been chosen and applied to the limb of a patient, typically the proximal bladder of the cuff is first inflated, after limb exsanguination, to a pressure intended to prevent blood flow past the cuff both proximally and into the exsanguinated limb. The anesthetic fluid mixture is then introduced into a vein in the limb as described previously. After a period of time sufficient for the anesthetic fluid mixture to induce analgesia in the limb below the proximal bladder of the cuff, the distal bladder is inflated to a pressure intended to prevent the flow of fluid past the cuff both proximally and distally. The distal bladder of the cuff is thus inflated over anesthetized tissue, thereby resulting in greater comfort for the patient for a greater period of time, thus potentially extending both the duration of surgical procedures which can be performed under IVRA and the number of patients for whom IVRA will be tolerable.
Surgical tourniquet systems of the prior art typically include an inflatable cuff for applying to a limb and an automatic pressure regulator for regulating the inflation pressure in the cuff near a reference level selected by an operator or determined automatically. Some tourniquet systems in the prior art have been associated with a number of reported hazards and problems which are not specific to IVRA, such as unnecessarily high pressures applied by the cuff leading to nerve injury and tissue damage beneath the tourniquet cuff, and unexpectedly low pressures applied by the cuff leading to sudden blood flow into the surgical site, complication of surgery, passive congestion of the limb, and hemorrhagic nerve infiltration. Additionally, the cuffs of prior art systems have design limitations which make the cuffs difficult to apply consistently to limbs of different shapes and sizes. These design limitations of many prior art inflatable cuffs and tourniquet systems lead to clinical situations in which the maximum pressure actually applied by a prior art cuff to a limb is significantly different than the pressure in the inflatable bladder of the cuff and thus pressure indicated by the tourniquet pressure display.
There are also specific hazards associated with the use of prior art tourniquet systems for IVRA because the pressure of liquid anesthetic agents introduced into limb veins has generally not been monitored in the prior art, which has led to excessive pressures in the veins distal to the tourniquet cuff, thus causing anesthetic agent to flow past the cuff and into the general circulation. This can lead to an ineffective regional anesthesia in general, and even to cardiac arrest and death in reported cases.
A serious problem associated with the use of prior art tourniquet systems in relation to the delivery of anesthetic agents for IVRA is that in the prior art the maximum pressure applied by the tourniquet cuff to the limb is determined and adjusted independently of, and without knowledge of, the delivery pressure of the anesthetic agent. Moreover, the anesthetic agent is delivered in the prior art manually at a maximum pressure that is highly variable and dependent on the variations in operator technique. Most significantly, in the prior art, the pressure of liquid in the veins distal to the cuff is not a function of the maximum pressure applied by the tourniquet cuff. Consequently, it cannot be assured that the applied pressure is sufficiently greater than the venous pressure distal to the cuff so that no anesthetic agent will flow unexpectedly past the cuff and into the general circulation.
Another problem associated with prior art tourniquet systems is that no provision exists for automatically adjusting the pressure applied by the cuff such that bleeding arterial vessels can be observed in the surgical wound prior to completion of surgery, while the anesthetic fluid mixture is simultaneously retained in the veins of the limb distal to the cuff. Bleeding vessels can be observed only if the applied pressure is reduced sufficiently to permit arterial inflow; however, at the same time the applied pressure must be great enough to stop venous outflow and thereby maintain anesthesia. Prior art tourniquet systems do not provide any methods for reliably establishing and maintaining this condition.
For reasons of improved patient safety, there is a clinical need for wider tourniquet cuffs which appear to stop blood flow distal to such cuffs at lower inflation pressures than narrower cuffs. However, a significant problem with prior art cuffs in general, and with wide cuffs in particular, is that reliable and consistent sealing of the bladders is difficult due to the high forces generated internally because the forces on the sealed seams of bladders are generally proportional to the total internal area of the cuff multiplied by the inflation pressure.
A number of problems are associated specifically with prior art pneumatic cuffs used for IVRA. First, prior art cuffs have generally employed two bladders which can be inflated or deflated independently. Each bladder of an IVRA cuff must be narrower than a conventional tourniquet cuff in order that the IVRA cuff can fit on the patient's limb and not obstruct the desired surgical site. Second, prior art tourniquet cuffs commonly employ a flexible thermoplastic stiffener to constrain the inflation of the bladder and direct cuff inflation inwardly toward the encircled limb. The incorporation of stiffeners into prior art cuffs stabilizes the cuff bladders across the bladder width and thus reduces the tendency of cuffs to roll longitudinally down a limb when the bladders are pressurized. However, certain problems and hazards are associated with the use of prior art stiffeners. First, the incorporation of stiffeners into prior art tourniquet cuffs has tended to cause such cuffs to form a substantially cylindrical shape when applied to a limb, resulting in a poor shape match for limbs that are non-cylindrical in shape in the region underlying the encircling cuff. The use of stiffeners in prior art cuffs has also tended to cause the cuffs to be more difficult to apply by operating room staff in a snug and consistent manner. Also, the incorporation of stiffeners into prior art cuffs has added significantly to the costs of manufacture of such cuffs. Finally, the incorporation of stiffeners into prior art cuffs has created difficulties when the cuffs are cleaned or resterilized because certain resterilization processes apply heat to the cuffs, distorting the shape of stiffeners which are commonly formed of flexible thermoplastic material, thus detrimentally affecting the subsequent ability of the distorted cuff to conform smoothly to the encircled limb.
The present invention overcomes many of the hazards and problems associated with technology described in the prior art and significantly reduces variations in the quality and safety of IVRA associated with variable knowledge, skill and experience of operators. Thus the present invention facilitates the increased use of IVRA for anesthesia of both upper and lower limbs.
An object of the present invention is to provide tourniquet apparatus for intravenous regional anesthesia which automatically relates the maximum pressure applied to a limb by the tourniquet cuff to the maximum pressure of fluid in the veins in a portion of the limb distal to the cuff, so that the flow of fluid past the cuff proximally and into the circulatory system can be automatically regulated and stopped in a safe and reliable manner, as desired by an anesthetist or surgeon.
Another object of the present invention is to provide tourniquet apparatus having automatic means for estimation of the lowest pressure which can be applied by the cuff of the tourniquet apparatus to a limb in order to stop blood flow distal to the cuff, where the cuff has design and physical characteristics which are substantially different than those of a conventional blood pressure cuff, where the cuff is applied with an undetermined degree of snugness at any location along the limb between its proximal and distal end, and where there may be a substantial mismatch between the shape of the encircled limb and the shape of the encircling cuff.
A related object is to provide tourniquet apparatus having wider and safer cuffs for reducing the probability that blood will unexpectedly flow past the cuff distally, for reducing the probability that anesthetic fluid mixture will unexpectedly flow past the cuff proximally, for reducing the probability that clinical staff will make errors in applying the cuff to the correct location anatomically, and for increasing the tolerance of the patient to the cuff when pressurized so that more patients can take advantage of intravenous regional anesthesia.
Another object of the present invention is to provide means for more consistent and safer exsanguination of a portion of the limb distal to the tourniquet cuff prior to introduction of anesthetic agent into a vein in that limb portion, by automatically regulating the pressure in a pneumatic exsanguinating cuff distal to the tourniquet cuff for a period of time, and by automatically and sequentially inflating the tourniquet cuff proximal to the exsanguinating cuff when sufficient blood has been exsanguinated from the surrounded portion.
The applicant is aware of the following United States Patents which are more or less relevant to the subject matter of the applicant's invention.
______________________________________ 4,469,099 9/1984 McEwen 128/327 4,479,494 10/1984 McEwen 128/327 4,605,010 9/1986 McEwen 128/686 4,770,175 9/1988 McEwen 128/327 4,869,265 9/1989 McEwen 128/774 4,321,929 3/1982 Lemelson 128/630 4,635,635 1/1987 Robinette-Lehman 128/327 4,781,189 11/1988 Vijil-Rosales 128/327 4,168,063 9/1979 Rowland 273/54B 3,164,152 1/1965 Vere Nicoll 128/87 4,667,672 5/1987 Romanowski 128/327 ______________________________________
The applicant is also aware of the following United States patent application which is more or less relevant to the subject matter of the applicant's invention. U.S. application Ser. No. 388,669; Title: Tourniquet for Regulating Applied Pressures; Art Unit: 335; Inventor: McEwen.